In advanced image sensor devices, a micro-lens is usually used to collect and focus light on a sensor. However, typical prior micro-lenses have a number of disadvantages. The overall height from the silicon surface of the semiconductor to the top layer, formed by a color filter process, is typically very large. This large height or distance might decrease the function of the micro-lens. Some micro-lenses are made from a positive photoresist. However, this positive photoresist has reliability issues in that the photoresist materials may yellow and are not capable of resisting high temperatures.
FIG. 1 illustrates a sectional view of a prior art semiconductor device 10 including a micro-lens 12. The semiconductor device includes a silicon-based substrate 30 in which a sensor 28 is formed such as a photodiode. An inter-metal dielectric layer 26, such as silicon dioxide, is provided over the silicon-based substrate 30. A silicon dioxide passivation layer 24 is provided over the inter-metal dielectric layer 26. A silicon nitride layer 22 is provided over the silicon dioxide passivation layer 24 and a planarization layer 20 is provided over the silicon nitride layer 22. A color filter layer 18 is provided over the planarization layer 20 and a micro-spacer 16 is provided over the color filter layer 18. The micro-lens 12 is formed over the color filter layer 18. The micro-lens 12 includes a convex shaped upper surface 14 which causes parallel light rays 2 to converge on and strike the sensor 28.
Abramovich, United States Patent Application Publication No. 2001/0010952A1, published Aug. 2, 2001, discloses a method of producing a color CMOS image sensor. Disclosed is a color image sensor that is formed on an n-type semiconductor substrate, having a p-well layer. An array of photodiodes and charged transfer regions are formed in a p-well layer, and are covered by silicon oxide or nitride film. A poly-silicon electrode is located over charged transfer regions such that it is surrounded by film. A photo-shielding metal layer is formed over the electrode, and a surface protective coating and a passivation layer are formed over the metal layer. The color filter layer is formed on the passivation layer, and an intermediate transfer film is formed over the color filter layer. A micro-lens for focusing light beams is formed from silicon dioxide or a resin material on intermediate transparent film. In the operation, light beams are focused by the micro-lens through the color filter layer such that they converge on the focal axis of the micro-lens to strike the photodiode, wherein photo energy from the light beams frees electrons in the photodiode. When a select voltage is applied to the poly-silicon electrode, these freed electrons generate a current in the charge transfer region. A sensor circuit associated with a color sensor then determines the amount of light received by the photodiode by measuring the amount of current generated in the charge transfer region. A dielectric layer may be provided and is etched to form the micro-lens. This may be accomplished using reactive-ion etching. The etching process is controlled such that a portion of the dielectric layer remains over the passivation layer. A silicon nitride layer may also be deposited and etched to form a micro-lens. Disclosed also is the process of depositing a photoresist portion and subsequently etching the photoresist layer to form a micro-lens.
Hsiao, et al., U.S. Pat. No. 6,417,022, issued Jul. 9, 2002, discloses a process of forming a multiplicity of micro-lenses for the color filters. A photodiode is first formed in the surface of a substrate, which also includes a series of metal connectors covered by a layer of passivation that forms an irregular upper surface. The irregular upper surface is then planarized to deposit a layer of dielectric material over the passivation layer. After the planarization step has been performed, a subsequent step is carried out to form a color filter layer providing red, green and blue color elements. As a final step, a micro-lens spacer is applied and followed by a step, which produces a micro-lens. Disclosed is a method in which a micro-lens material is patterned into at least three separate embryos performing three separate micro-lenses of long focal length. Each of the micro-lens embryos may be divided into nine discreet regions of different sizes. After the embryos are patterned by standard photolithographic methods, the embryos are reflown in a reflow process at a temperature of about 160 degrees Celsius, forming a micro-lens.
Hsaio, et al., U.S. Pat. No. 6,582,988, issued Jun. 24, 2003, discloses a method of making a micro-lens including providing a silicon substrate having a plurality of CMOS devices. A non-photosensitive planarization layer is deposited over the substrate. A color filter layer is placed over the top of the planarization layer with individual red/green/blue (RGB) sub-section pixel areas. A negative type photo-resistant material is utilized for the planarization/passivation layer formed directly over the color filter layer. One material disclosed as suitable for that application is a novolac/acrylic matrix supplied as CT-3050L by Fuji-Olin Corporation. A positive type photo-sensitive material is applied over the planarization/passivation layer. One embodiment disclosed as suitable for the positive type photo-sensitive material is a mixture of melamine resin and generic novolac based resin. Thereafter, a plasma etch step is utilized with oxygen plasma to etch through exposed regions of the planarization layer to define the micro-lens array. After the plasma etch step, the micro-lens array undergoes a hard bake reflow operation to produce the micro-lens.
The present invention provides alternatives to the prior art.